Truck boxes or trailers are commonly equipped with rear end walls referred to as tailgates. The tailgate is mounted on hinges at it's bottom edge and latched at the top edge in a manner that allows the tailgate to be unlatched and folded down to accommodate loading. When the truck box is empty, there is no need to close the tailgate except that in the opened position it is readily damaged. Thus it is most common to have the tailgate closed except when loading or unloading.
A very high percentage of the miles traveled by many trucks is with the truck box empty (most notably pickup trucks which are often used principally as transportation vehicles and only occasionally for hauling). Whereas the truck cab is readily designed to minimize wind resistance during high speed travel (thereby saving on fuel consumption) the truck box is designed to maximize the loading capacity and is substantially a rectangular box. The tailgate in the closed position presents a flat surface to the airflow and thus a substantial resistance. It functions to resist forward travel of the vehicle with a resulting increase in fuel consumption.